Heat Transfer by Radiation
Heat energy can travel in the form of waves, just like light. These waves (both heat and light, and many other types of waves) are called electromagnetic waves. Energy travelling in this form is called radiation. Just like light can travel – in different media (like air, water, etc.) and in vacuum, heat radiations can also travel in different media and in vacuum.
All bodies give out energy that travels in the form of radiation (much like light) through space. Energy from the sun reaches us through radiation. Bodies that absorb this radiation become hot. All objects radiate heat energy, and this property is used in infrared photography.
This helps us ‘see’ objects in complete darkness. Some animals can ‘see’ in the infrared. For example, rattle snakes have special detectors with which they can ‘see’ in absolute darkness by detecting the heat radiated by warmblooded animals.
The amount of heat energy that is absorbed by a body depends on its colour. Bodies that are black absorb more of radiated heat than white bodies. We feel more comfortable wearing white and light-coloured clothes in summer. This is because white clothes absorb comparatively less amount of heat than dark clothes, and therefore keep us relatively cooler.
The reverse applies in winter; we are more comfortable in dark clothes. This is because we need to absorb as much heat as possible from our surroundings to keep ourselves warm, and dark clothes absorb more heat than light-coloured ones. The reverse is also true. Black and dark bodies radiate more heat than light-coloured bodies.
Practical Applications of Radiation
Fig. Room heater Fig. Solar panel
Heat radiations travel in vacuum and in air just like light rays. That is why electric room heaters have mirrors behind the heating coil (Fig.). This reflects the heat radiated by the heating coil to the front of the heater. This way the heat radiated in the direction behind the heater is not wasted, but redirected to the front.
Solar panels (Fig.), used in households for heating water, are designed to maximize the absorption of heat from the sun. A black metal sheet is used to increase heat absorption.
Aim: To show radiation of heat
Materials needed: A source of heat, like a room heater or an electric bulb, and adult supervision
- Switch on the room heater or the electric bulb.
- Wait for a few minutes, and put your hand close to it without touching.
Observation: You will be able to feel the heat of the heater/bulb even when you do not touch it.
Conclusion: Heat energy is reaching your hand from the heater/bulb in the form of radiation.
In the above activity, you can confirm that the heat energy reaching your hands is being carried by radiation and not convection. Hold your hand a little below the bulb. The heat which you feel below the bulb is mostly due to radiation as hot air always rises up.